The invention relates to a hydrophobicizing agent for gypsum-plaster-bound construction materials, its use and a process for its preparation.
Gypsum plaster is a construction material available at low cost and in large quantities. Besides the use of gypsum plaster in jointing compositions, gypsum mortars are especially used for internal renders and for coatings. However, due to their sensitivity to water, renders which comprise calcined gypsum as binder are used only for internal renders exposed to the conditions usual for these, but not for high-humidity areas or for external renders. Construction materials based on gypsum plaster have to be adequately hydrophobicized if they are to be used externally or in wet areas. The prior art therefore frequently makes its organosilicon compounds with gypsum plaster compositions to hydrophobicize the same.
DE-A 2740049 (GB-A 1581396) describes a process for preparing a water-repellent gypsum plaster product, in which pulverulent gypsum plaster is mixed with a linear monomethylpolysiloxane in the absence of water. A disadvantage of this process is that a relatively large amount of monomethylpolysiloxane is required for the hydrophobicization, and the usage properties of the resultant pretreated pulverulent gypsum plaster becomes poorer.
It is known from EP-A 171018 (U.S. Pat. No. 4,643,771) that hydrophobicized plaster moldings can be produced by adding a foam prepared from alkylhydropolysiloxanes, surfactant and water to the aqueous plaster mix. A significant disadvantage of this process is that in practice when plasterboard is produced, the amount of plaster mix metered is regulated, inter alia, by varying the amount of foam added. The hydrophobicization of the final product is therefore not uniform.
DE-A 4128424 relates to a process for the hydrophobicization of gypsum plaster compositions, where the alkylhydropolysiloxane used as a hydrophobicizing agent is an oil which is mixed intimately with the water used for mixing, and then the pulverulent gypsum plaster is added. U.S. Pat. No. 3,455,710 discloses that alkylhydropolysiloxanes in the form of their aqueous emulsion can be added to gypsum plaster as a hydrophobicizing agent for the same purpose. The two last-named processes have the disadvantage that the hydrophobicizing agent has to be admixed by, and therefore also metered by, the user. Variations are therefore certain to occur. In addition, when the material is admixed in the form of an aqueous emulsion, consideration has to be given to the relatively high level of hydrogen cleavage with resultant fall-off in the effectiveness of the polysiloxane.
The process described in DE-A 3704439 provides a marked improvement by using the hydrophobicizing agent in the form of a pulverulent product which is obtained in the form of a water-redispersible powder by spray drying a mixture made from water, from alkylhydropolysiloxane and from a water-soluble film-forming polymer having a specific cloud point, for example, polyvinyl alcohol. This powder can then be added to the gypsum plaster in order to hydrophobicize the same. Disadvantages are the not inconsiderable production cost of the powder, and also its unsatisfactory storage stability. This carries with it the risk of a hydrogen explosion or dust explosion prior to incorporation into the pulverulent gypsum plaster, since there is the possibility of hydrogen cleavage.
The object was therefore to develop a hydrophobicizing agent which can be used for gypsum-plaster-bound construction materials and which permits uniform incorporation of the hydrophobicizing agent into the pulverulent gypsum plaster, or into the pulverulent gypsum plaster mixture. In addition, the pulverulent hydrophobicizing agent, both on its own and in the ready-to-use mixture, should have good storage stability and pose a minimal risk of explosion.
The invention provides a hydrophobicizing agent for gypsum-plaster-bound construction materials comprising a substrate based on pulverulent organic or inorganic solids with a BET surface area  greater than 5 g/m2 and comprising one or more organopolysiloxanes having Si-bonded hydrogen (organohydropolysiloxanes) and built up from units of the general formula HaSiR1bO(4-a-b)/2, where the radicals R1 are identical or different monovalent hydrocarbon or hydrocarbonoxy radicals having from one to 15 carbon atoms, or are hydroxyl groups, with the proviso that at least one Si-bonded hydrogen atom per molecule is present, and a takes values from 0 to 3, and b takes values from 0 to 3.
Suitable substrates from the group of inorganic substrates are silicon dioxide such as silica gel or silica, aluminum oxide, titanium dioxide, silicates and clays. Preference is given to silica and aluminum oxide, particularly preferably aluminum oxide and silica prepared by precipitation using solution chemistry, or pyrogenically by flame hydrolysis. Suitable substrates from the group of organic substrates are starch powders, cellulose powders and polymer powders. Preference is given to polyvinyl chloride powders, in particular suspension PVC powders, polystyrene powders, polyamide powders and polyvinylpyrrolidone powders. Aluminum oxide and silica prepared by precipitation using solution chemistry are most preferred.
Preference is also given, to pulverulent inorganic or organic substrates with a BET surface area of from 20 to 400 m2/g, measured by the BET method to DIN 66131 and DIN 66132. Particular preference is given to substrates which do not promote hydrogen cleavage in the organohydropolysiloxanes, i.e. those whose surface is neutral to slightly acidic, particularly with a pH of from 5.5 to 7.0 (5% strength by weight suspension in water). Particular preference is also given to substrates with good adsorption properties, i.e., liquid uptake  greater than 10% by weight, based on the weight of the substrate.
Examples of hydrocarbon radicals having from 1 to 15 carbon atoms as radical R1 are alkyl radicals such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, tert-pentyl, n-hexyl, n-heptyl, n-octyl, isooctyl or 2,2,4-trimethylpentyl, n-nonyl, n-decyl, n-dodecyl; alkenyl radicals, such as vinyl and allyl; cycloalkyl radicals such as cyclopentyl, cyclohexyl, cycloheptyl and methylcyclohexyl; aryl radicals such as phenyl, naphthyl, anthryl and phenanthryl; alkaryl radicals such as o-, m-, p-tolyl, xylyl and ethylphenyl; aralkyl radicals such as benzyl and alpha- and beta-phenylethyl. Where appropriate, the hydrocarbon radicals having from 1 to 15 carbon atoms may also have halogen substitution. Examples of such cases are fluorine-, chlorine-, bromine- or iodine-substituted alkyl radicals such as 3,3,3-trifluoro-n-propyl, 2,2,2,2xe2x80x2,2xe2x80x2,2xe2x80x2-hexafluoroisopropyl, heptafluoroisopropyl and haloaryl radicals such as o-, m- and p-chlorophenyl. Examples of hydrocarbonoxy radicals are the abovementioned C1-C15-hydrocarbon radicals bonded to the silicon atom via a bivalent oxygen atom.
Particularly preferred radicals R1 are the unsubstituted C1-C6-alkyl radicals, in particular methyl, and phenyl. In another preferred embodiment, not more than 5 mol % of the radicals R1, and particularly preferably none of these radicals, are hydrocarbonoxy radicals or hydroxyl groups. The variable a is preferably from 0.01 to 2, in particular from 0.1 to 1, and b is preferably from 1.0 to 2.0, in particular from 1.0 to 1.8. The average viscosity of the organohydropolysiloxanes is not more than 1000 mm2/s, preferably from 5 to 1000 mm2/s, in particular from 5 to 100 mm2/s, in each case at 25xc2x0 C.
Most preference is given to organohydropolysiloxanes of the general formula (CH3)3SiO(SiHCH3O)y(Si(CH3)2O)xSi(CH3)3, where x is zero or an integer and y is an integer not less than 3, and y is at least three times x, preferably at least 13 times x, and the sum of x and y is an integer such that the average viscosity of these organohydropolysiloxanes is not more than 1000 mm2/s, in particular from 5 to 100 mm2/s, in each case at 25xc2x0 C.
The hydrophobicizing agent generally comprises from 20 to 75% by weight of organohydropolysiloxanes, based on the total weight of substrate and organohydropolysiloxane.
To prepare the hydrophobicizing agent, the organohydropolysiloxanes are used in the form of a liquid, preferably without adding solvent. The substrate generally forms an initial charge, and the liquid organohydropolysiloxane is then metered in and is applied to the substrate by mixing with the substrate. The organohydropolysiloxanes are generally applied to the substrates at room temperature, and may be applied in conventional devices suitable for mixing powders. Examples of these which may be mentioned are continuously operating or batchwise-operating ribbon blenders, double-helix mixers, blade mixers, plowshare mixers, fast- or slow-speed paddle mixers, vortex screw mixers, and pan mixers. Mixers with rotating mixing vessels may also be used, for example Rh{umlaut over (n)}rad mixers, tumbling mixers, double-cone mixers and V-blenders. The organohydropolysiloxanes may also be applied to the substrates by spraying the liquid organohydropolysiloxanes onto the pulverulent substrates.
The hydrophobicizing agent is suitable for hydrophobicizing all types of gypsum plaster. The types of gypsum plaster include alpha- and beta-hemihydrate (CaSO41/2H2O) in the form of calcined gypsum, stucco plaster or modeling plaster. Other common types of gypsum plaster are flooring plaster, Keene""s cement, and anhydrite, and also the calcium sulfate produced in flue gas desulfurization.
The gypsum plaster composition may moreover comprise the usual aggregates and additives. Usual aggregates for gypsum mortar are from 1 to 30% by weight of hydrated lime, and also the usual amounts, preferably from 5 to 80% by weight, of inert fillers such as calcium carbonate, dolomite, calcium sulfate and/or quartz sand. All of the % by weight data are based on the total weight of the pulverulent gypsum plaster composition. Examples of additives which improve the usage properties of the gypsum plaster composition or the properties of products manufactured therewith are salts of long-chain fatty acids such as calcium stearate and sodium oleate, silicone buildings-preservation materials, fungicides, fibrous materials, such as cellulose fibers, accelerators such as dipotassium hydrogensulfate and aluminum sulfate, retarders such as proteins and tartrates, plasticizers such as lignin sulfates, adhesion promoters such as starch, and binders, as in water-redispersible polymer powders.
To hydrophobicize the gypsum plaster, the hydrophobicizing agent is mixed with the gypsum plaster in suitable mixers for example in the above-mentioned mixing assemblies suitable for mixtures of powders. Another possible procedure is to mix the hydrophobicizing agent with the additives for modifying gypsum plaster compositions, and then adding the same to the gypsum plaster. For example, the hydrophobicizing agent may be added during the spray drying of aqueous polymer dispersions to prepare water-redispersible polymer powders. A preferred process here is to add the hydrophobicizing agent to the dried polymer powder by continuously metering the hydrophobicizing agent into the spray dryer, where appropriate with other additives such as antiblocking agents and antifoam.
The amount of the hydrophobicizing agent added to the gypsum plaster composition is such that, based on 100 parts by weight of gypsum plaster in the hydrophobicized gypsum plaster composition, from 0.05 to 5 parts by weight, preferably from 0.2 to 2 parts by weight, of organohydropolysiloxane are present.
The gypsum plaster compositions hydrophobicized according to the invention are especially suitable for use as a gypsum mortar for troweling compounds, joint fillers, flowable screeds, jointing compositions or adhesive mortars. Examples of other applications are renders or stucco work specifically in the outdoor sector. Examples of other uses are the production of plasterboard or plaster moldings.